1. Field of the Invention
The present invention relates to washing machines. More particularly, though not exclusively, the present invention relates to a method and apparatus for controlling the liquid level in a washing machine.
2. Problems in the Art
A typical prior art washing machine controls the fill level by use of an air dome. A typical prior art air dome includes a pressure switch in communication with a sealed tube. The open end of the tube is in communication with the bottom of the wash tub and extends upward toward the pressure switch. As water enters the wash tub and the sump, water will partially enter the opening of the air dome hose and, as the water level rises, the air pressure in the sealed tube increases and ultimately trips the pressure switch. When the pressure switch trips, the washing machine stops filling. One example of a typical air dome is disclosed in U.S. Pat. No. 3,397,716 which is incorporated by reference herein. The air dome described above does not form a part of the present invention.
One major problem with prior art air domes occurs when water is allowed to enter the air dome hose. When this happens, the fill level of the washing machine increases since a greater fill level is required to trip the pressure switch. When water does enter the air dome hose the water does not always come out due to the vacuum caused by the sealed hose. This is similar to placing a finger on the end of a straw to trap liquid in the straw. The fill level required to trip the pressure sensor will increase by an amount equal to the length of the water column trapped in the air dome hose. In other words, if six inches of water are trapped in the air dome hose, the level at which the washing machine will quit filling is increased by six inches. It is possible that the increased fill level will cause the washing machine to overflow.
A common way that water becomes trapped in the air dome hose is when a small amount of water is left in the sump at the bottom of the wash tub. When the washing machine is tipped, for moving or maintenance for example, water will flow into the hose and will not come out. When this happens, the next time the washing machine is filled, the water level required to trip the pressure switch will be greater by an amount equal to the amount of water trapped in the air dome hose.
The prior art teaches to position the air dome outside of the wash tub because of the effects of varying water temperatures. If the air dome is within the wash tub, the temperature of the water will affect the temperature of the air in the air dome. For example, if the water is hot, the air in the air dome will heat and expand, changing the amount of water required to trip the pressure sensor. The prior art therefore teaches away from locating locate the air dome anywhere but outside of the wash tub.